1. Field of the Invention
The present invention relates to an ink composition and particularly to a water-base pigment ink composition suitable for printing on specialty paper for ink jet recording.
2. Background Art
Ink jet recording is a printing method wherein droplets of an ink are ejected and deposited on recording media, such as paper, to perform printing. An innovative advance of a recent ink jet recording technique has made it possible to realize the production of images having quality comparable substantially favorably with images produced by silver salt photography or images yielded by high-definition printing realized by offset printing. To realize such high-quality images, ink jet recording media having a high level of gloss comparable to photographic paper, art paper and the like used in the field of silver salt photography and offset printing have been developed. Media comprising an ink-receptive layer containing a porous pigment such as silica provided on a substrate such as paper or a film have been mainly used as this type of high-gloss recording media.
The ink used in recording characters or images on the above high-gloss recording media is generally a water-base ink comprising water as a main component and a colorant, a resin emulsion, and other various additives added to the water. Dyes or pigments may be used as the colorant. However, pigments are superior to other colorants in weatherfastness such as lighffastness, gasfastness, waterfastness, and moisturefastness. Therefore, in recent years, the development of pigment inks utilizing properties of pigments has been advanced. Further, water-based pigment inks, which contain a resin emulsion for improving the fixation of the pigment onto the recording medium, have become used.
The use of the recording medium having an ink-receptive layer in combination with the resin emulsion-containing pigment ink composition can realize good rubbing/abrasion resistance and good scratch resistance of the formed images. Specifically, upon printing of the ink composition onto the recording medium, the ink-receptive layer absorbs the solvent component and allows the pigment particles and the fine particles of the polymer in the resin emulsion to stay on the surface of the recording medium. The reason for this is as follows. On the surface of the recording medium, since the amount of the solvent component is reduced, the density of fine particles of the polymer is increased and a film forming reaction by the fine particles of the polymer is allowed to proceed to form a film which allows the pigment to be fixed onto the surface of the recording medium.
Further, ink compositions containing a water-soluble resin have also been proposed. For example, Japanese Patent Laid-Open No. 18467/1992 discloses a water-based ink containing a specific carbon black and a water-soluble resin having an acid value of not less than 100 and a weight average molecular weight of 3,000 to 7,000 in an aqueous medium. The claimed advantage of this water-base ink is that precipitates are less likely to occur even after storage for a long period of time. Further, Japanese Patent Laid-Open No. 70254/1995 describes a water-soluble resin which has been produced by neutralizing, with a basic compound, a copolymer having an high acid value produced by copolymerizing 21 to 99% by weight of an ethylenically unsaturated carboxylic acid monomer with other monomer in the presence of polyvinyl alcohol.
Further, ink compositions, into which various waxes have been incorporated mainly for improving the fixation onto a recording medium and the waterfastness, are also known in the art. For example, Japanese Patent Laid-Open No. 370166/1992 describes a water-based ink composition containing a pigment, polyvinylpyrrolidone, and wax. In this water-based ink composition, after printing onto a recording medium, heat treatment should be carried out at a temperature at or above the melting point or softening point of the wax.
There is a still strong demand for high quality of images yielded by ink jet recording. Further, the development of ink compositions for ink jet recording, which can realize high-quality images, has been desired.